Security processors are generally hardware components containing confidential information that only legitimate users can use. Examples of such confidential information include cryptographic keys and access rights. To preserve the confidentiality of this information, these processors are designed to be as robust as possible against attack attempts by computer hackers. In one example, a security processor is a chip card equipped with an electronic processor.
Security processors are subjected to different types of attack. Some of these attacks are aimed at extracting or determining the confidential information contained in the security processor. To this end, a multitude of attacks have been developed. For example, certain of these attacks seek to obtain an abnormal functioning of the security processor by making it process messages built by computer hackers. Other more invasive methods try to disturb the functioning of the security processor at key moments in its operation by playing on its supply voltage or again by means of a laser beam directed towards the security processor.
Other types of attack do not seek to extract or determine the confidential information contained in the security processor but consist simply of the abusive use of this security processor. For example, in pay television, control sharing and card sharing come under this type of attack. Control sharing consists of the sharing of the control word deciphered by the security processor among several receivers. These receivers can then decipher the scrambled multimedia contents with this control word, even though only one receiver has actually paid for a subscription.
In card-sharing, the same security processor is made to decipher several enciphered control words coming from different receivers. As above, all these receivers can then descramble the scrambled multimedia contents whereas only one of these receivers is entitled to access the content.
To combat these attacks, there are known ways of detecting attack attempts and, in response to this detection, to execute countermeasures.
One example of a method for detecting attack attempts and for executing countermeasures in response is described in the patent application EP 1 575 293.
A countermeasure is an action aimed at preventing an attack against the security processor from being long-lasting or successful. There are a large number of countermeasures that can be executed by a security processor. These measures range from a simple increase in security measures in the security processor up to the definitive and irreparable blocking of the security processor, which then becomes unusable.
Methods for detecting an attack attempt have already been proposed. These methods comprise: measuring several different events occurring independently of one another in the absence of attack attempts, followed by comparing each measurement with a predetermined respective threshold to detect the presence or absence of an attack attempt.
However, a difficulty arises from the fact that the events representing an attack attempt can also occur when there is no attack attempt. It is therefore necessary to prevent the production of false detections of attack attempts because these false alarms may result in the untimely execution of countermeasures, which then inconvenience the legitimate user of the security processor. For this reason, there are known ways of choosing a far higher value for the predetermined threshold than the values of all the measurements that can be obtained when there are no attacks. However, raising the predetermined threshold makes certain attacks undetectable or slows down the detection of an attack attempt.